Geophysical constraints on the variscan crust of sw-iberia

Abstract

The focal point of this Thesis is obtain the crustal composition of SW-Iberia using as a geophysical data two densely spaced wide-angle seismic reflection/refraction data (IBERSEIS Wide-Angle). The study area is the south westernmost exposed part of the Variscan Belt in Europe and offers the opportunity to study the Variscan orogeny (480 - 290 Ma), that in the region is formed by the collision of three continental blocks: the South Portuguese Zone (SPZ), the Ossa-Morena Zone (OMZ) and the Central Iberian Zone (CIZ). The two wide-angle surveys were designed to determine the physical properties in SW-Iberia and complement the normal incidence seismic survey IBERSEIS (Simancas et al., 2003). Both transects crossed the 3 main tectonic units in the area and their tectonic contacts. Transect A was 300 km long and Transect B was 250 km long. The data set was recorded by vertical component recorders with a station spacing of 400 m and 150 m for Transect A and B, respectively. The close station spacing allows the identification and correlation of more phases, as well as increases the lateral continuity of them. It increases the interpretative complexity as more events are identified. As a result a complex P-wave velocity model has been obtained by forward modeling. Some S-wave arrivals were also identified on the shot gathers that were modeled. From the P- and S-wave velocity models, the Poisson's ratio distribution for the crust was calculated. From the P-wave velocities the crustal density models were derived and constrained by the geopotential fields response. As a result, two density and two thermal lithospheric models and the topography of the lithosphere-astenosphere boundary were obtained. The P- and S-wave velocity, Poisson's ratio and density distribution for the crust constrain the nature of the structures imaged by the IBERSEIS transect. Linking the estimated P-waves velocities with rocks types results in a broad range of possible lithologies. The integration with other geophysical observables as Poisson's ratio and density narrowed the range of possible rock types in order to get a sound interpretation of the crustal model. The P-waves velocity, Poisson's ratio and density models feature high values at mid crustal depths, coincident with the position and shape of the Iberseis Reflective Body (IRB) imaged by IBERSEIS. The compatible lithologies for this body are rocks of mafic composition. Modeling also shows the nonuniqueness of the IRB, since another body with the similar physical characteristics is mapped to the SWS of the IRB. Furthermore, the reflected phases that image these volumes or structures that are characterized by high velocity values, feature a coda indicating the complex nature of these areas. Lensing consisting of lamellae of different rock types featuring small changes in physical properties can account for the observed coda recorded. The size of the lenses is not big enough to be resolvable by the seismic technique. Therefore, the measured physical properties have to be considered as the average estimates of these laminated areas. This geophysical study has also revealed an upper mantle reflector identified at far offsets in the wide-angle shot records. The modeling of this arrival indicates that it could correspond to a gradient zone consistent with the phase transition from spinel-herzolite to garnet-lherzolite, i.e., the Hales transition zone. The mafic intrusions at mid crustal levels that cross the tectonic limits, the derived nature of the rocks, the uplift of the LAB beneath the IRB, jointly with the observed surface geology, suggest a new and unique tectonic scenario involving a post collisional intraorogenic intrusion of mantle derived rocks.